1. Field of the Invention
The present invention relates to a position detector for a moving vehicle, and more particularly to a steering control system for traveling a moving automobile, an unmanned mobile and carrying device in a factory, or a vehicle for use in agriculture, civil engineering and the like along a predetermined traveling course.
2. Description of the Prior Art
Heretofore, as a system for detecting a present position of a moving body such as the moving vehicles described above, there has been proposed a system provided with a means for scanning a light beam emitted from a moving body in the circumferential direction or every azimuthal direction centering around the moving body, light-reflecting means for reflecting the light beam in the direction of incident light and secured at least three positions apart from the moving body, and a beam receiver means for receiving light reflected from the light-reflecting means (the Japanese Patent Laid-open No. 67476/1984).
In the above mentioned prior art, differential azimuths between adjoining two of three light-reflecting means centering around the moving body are detected on the basis of received beam Output of the light-receiving means, and then a position of the moving body is determined based on the detected differential azimuths and positional information of the light-reflecting means which have been previously set.
In such a prior art system, if a moving bodY having a beam-receiving means mounted thereon is positioned in the neighborhood of the central portion of a triangle with the respective light-reflecting means secured at the three positions as the vertexes, the position of the moving body can be detected with a fairly high precision. However, there was a problem that, in a position apart from the neighborhood of the central portion of the triangle, it was difficult to obtain a high measuring precision.
As a countermeasure therefor, for instance, as described in the Japanese Patent Laid-open No. 14114/1985 or No. 15508/1905, there have been proposed methods for measuring the position of a moving body wherein reference points are set on the vertexes of a quadrangle surrounding the area in which the moving body travels, optimum three reference points of the four reference points with which a high measuring precision can be expected are selected, and the positional detection of the moving body is performed on the basis of the positional information of the selected three reference points and the azimuths of the reference points viewed from the moving body.
In the method described in the Japanese Patent Laidopen No. 14114/1985 of the above-mentioned methods for measuring the position of the moving body, first the position of the moving body is temporarily operated on the basis of the azimuths of the four reference points, the reference point Of said four reference points nearest the moving body is determined, and the position of the moving body is operated again on the basis of the positional informations and azimuths of the reference point nearest the moving body and two additional reference points at both sides of the nearest reference point, a total of three reference points.
On the other hand, also in the method described in the Japanese Patent Laid-open No. 15508/1985, based on the temporarily determined position of the moving body, a triangle surrounding the moving body is found, three reference points which are the three vertexes of this triangle are selected, and the position of the moving body is redetermined from the positional information and azimuths of the selected three reference points.
Thus, in the prior art, there was a problem that the procedure for selecting the reference points to be used for operating the position of a moving body (hereinafter referred to as a moving vehicle) was complicated.
In order to accurately detect the position of the moving vehicle, accurate positions of the above reference points must be set in an arithmetic operation device in advance. However, an initial setting of the reference points may sometimes be deviated from the real positions thereof. In that case, an error occurs in the detection of the position of the moving vehicle. And the error has a directionality in such a way that a detected position of the moving vehicle is always deviated, for example, to the right of the real position when a reference position to be used for detection is fixed. Hence, it is often confined within the tolerable range of small errors.
However, when some of the optimum reference points based on which high measurement accuracy can be expected, are selected from many reference points and the selected reference points are switched depending on the current position of the moving vehicle, the direction or polarity of the error in the detected position data of the moving vehicle may be inverted (e.g., the deviation in the right direction is inverted to a deviation in the left direction) at the time of said switching. In this case, an error two times the magnitude of the error present before the reference points have been switched will occur, so a countermeasure therefor is required.
Furthermore, the countermeasure has been needed due to the fact that the detected positions of the moving vehicle become unsuccessive by the switching of the reference points, and the detection data is discontinued.